ALSA: usb-audio: Initialize Dell Dock playback volumes

In the early commit adcdd0d5 ("ALSA: usb-audio: Skip volume
controls triggers hangup on Dell USB Dock"), we add the mixer quirks
for Dell dock to skip two mixer FU's for playback.  This supposed that
the device has always the proper initial volume, but it doesn't seem
always correct.

This patch adds the explicit initialization of the volumes to the
fixed 0dB at the device probe time.  Also, such a fixup is needed
after the resume, so a new function is hooked to the resume callback
as well.

Bugzilla: http://bugzilla.suse.com/show_bug.cgi?id=1089467


Signed-off-by: Takashi Iwai <tiwai@suse.de>